1. Field of the Invention
The present invention relates generally to wafer processing fixtures. Specifically, the present invention relates to silicon support fixtures having multiple silicon members secured together.
2. Technical Background
In the fabrication of semiconductor wafers it is often necessary to hold the wafers in precise positions during various processing steps. Relatively large and complex structures such as xe2x80x9cboatsxe2x80x9d or xe2x80x9ctowersxe2x80x9d are typically employed to that end. One example of such a structure is described in U.S. Pat. No. 5,492,229 to Tanaka et al. The Tanaka et al. patent is directed to a vertical boat for holding a plurality of semiconductor wafers. The boat includes two end members and a plurality of support members. In one embodiment, the support members are formed from pipe members cut vertically to provide a long plate member having a cross section of a quarter-circular arc. In another embodiment, the support members are formed from pipe members cut vertically to provide a long plate member having a cross section of a semicircular arc. The Tanaka et al. patent lists as potential materials for its boats the following: silica glass, silicon carbide, carbon, monocrystal silicon, polycrystal silicon, and silicon carbide impregnated with silicon. The various components are to be welded together if made from silica glass; otherwise, xe2x80x9cthey may be assembled in a predetermined mannerxe2x80x9d.
U.S. Pat. No. 5,534,074 to Koons is directed to a vertical boat for holding semiconductor wafers. The boat includes a plurality of rods having slots cut along their lengths. The configuration of the slots is intended to reduce shadowing on wafers placed within the boat during processing. The rods are cylindrical, and are specified as being made from fused quartz, although xe2x80x9cany known material suitable for holding wafers may be used.xe2x80x9d
U.S. Pat. No. 4,872,554 to Quernemoen shows a reinforced carrier for silicon wafers and the like. The carrier includes side components consisting of tubular rails with wafer spacing and supporting teeth projecting therefrom. The rails are made from plastic, and may be provided with rigid inserts for stiffening purposes. The teeth can be integrally molded with, or fused to, the rails.
U.S. Pat. No. 5,752,609 to Kato et al. is directed to a wafer boat including a plurality of rods arranged to support ring members. A plurality of wafer supporting pieces are associated with the ring members, and include angular projections for contacting the wafers. The Kato et al. patent also illustrates a wafer boat having a plurality of cylindrical quartz rods having wafer support recesses formed therein.
The theoretical advantages provided by pure silicon structures are well known. Conventional towers and boats are typically made from quartz or silicon carbide, which introduce contamination and become unstable at higher temperatures. By fabricating wafer holding structures from the same materials as the wafers themselves, the possibility of contamination and deformation is minimized. The structure would react to processing temperatures, conditions, and chemistry in exactly the same way that the wafers would, thus greatly enhancing the overall effective useful life of the structure.
Unfortunately, standard assembly of silicon structures in a xe2x80x9cpredetermined mannerxe2x80x9d as set forth in Tanaka et al. is one of the reasons that pure silicon has not gained wide acceptance as a material for structures such as boats and towers. The difficulties of working with monocrystalline silicon, polycrystalline silicon, and virgin polycrystalline have led to the development of structures such as that shown in Tanaka et al., wherein, when considering monocrystalline silicon as the material of choice, the connections between the support members and the end members are not described at all, and the only specifically described method of fabricating support structures involves cutting extruded tubular members. Such support structures are inherently less stable than those made from more traditional and easily-worked materials such as quartz or silicon carbide.
Similarly, the patents to Koons, Quernemoen, and Kato et al. fail to address the specific problems of providing a strong, reliable wafer support structure that reduces shadowing and contamination. The projections and slots described in these patents, while effective to some extent, are either not suited for fabrication from materials such as silicon, or require a relatively large cross-sectional area to provide stable and precise wafer support.
Silicon is perceived as being extremely fragile and difficult to fuse. Due to these perceptions, known silicon structures are widely believed to be delicate at best, and unreliably flimsy at worst. Consequently, they have failed to receive broad commercial acceptance.
It can thus be seen that the need exists for a strong, reliable support member for wafer processing fixtures that will reduce shadowing and contamination while providing stable and precise wafer support.
A method of securing a first silicon member to a second silicon member to form at least a part of a silicon wafer processing fixture is disclosed, as is the silicon wafer processing fixture itself. The method includes the step of providing a first silicon member with an outwardly-extending attachment element. A second silicon member is provided, with an attachment element receiving portion adapted to at least partially enclose the attachment member. The attachment element is then fixedly secured within the attachment element receiving portion.
In an embodiment, the step of fixedly securing the attachment element can be accomplished by providing a first transverse bore in the attachment element and a second transverse bore in the attachment element receiving portion. The first and second transverse bores are coaxial with one another, and adapted to receive a locating pin. Once the first and second bores are coaxially aligned with one another, the locating pin is secured in the first and second transverse bores. The pin can be provided with a length slightly greater than the combined length of the first and second bores, in which instance the pin can be secured in the following manner. First, the locating pin is inserted in the aligned bores such that a portion of the locating pin extends outwardly from an outer limit of the first and second bores. Next, the extending portion of the locating pin is machined off flush with the outer limit of the first and second bores. Alternatively, the locating pin can be provided with an outer diameter substantially equal to an inner diameter of the first and second bores, in which instance the pin can be secured in the following manner. First, the locating pin is cryogenically frozen, causing the locating pin to contract. Next, the locating pin is inserted in the aligned bores while maintaining the bores at ambient or higher temperature. Then, the locating pin is caused to expand by allowing the locating pin to return to ambient temperature.
In an alternative securing step, energy can be applied to at least one of the attachment element and the attachment element receiving portion to fuse the attachment element to the attachment element receiving portion. In an embodiment, the attachment element receiving portion of the second silicon member is provided with an access bore. Laser energy is applied through the access bore to form a tack weld between the attachment element and the attachment element receiving portion. Alternatively, the attachment element and the attachment element retaining portion can be substantially coextensive. Laser energy can be applied to an area adjacent to both the attachment element and the attachment element receiving portion.
In yet another embodiment, the first silicon member is provided with a peripheral ridge at its terminal end. The second silicon member is provided with a peripheral trench at its terminal edge. Laser energy is applied to cause the peripheral ridge of the first silicon member to melt into the peripheral trench of the second silicon member. Laser energy is applied to heat the ridge to a temperature of approximately 1450xc2x0 C. for approximately 3 minutes, or until the silicon melt has filled the peripheral trench.
In still another embodiment, the step of fixedly securing the attachment element within the attachment element receiving portion includes the step of applying a high-bond, non-contaminating adhesive between the attachment element and the attachment element receiving portion.
The attachment element can be generally cylindrical, with the attachment element receiving portion being formed as a cylindrical bore having a diameter and length corresponding to those of the attachment element. The first and second silicon members can be formed from monocrystalline silicon, polycrystalline silicon, or virgin polysilicon.
A silicon wafer processing fixture is also provided. The fixture includes a generally elongate silicon support member having an attachment element extending outwardly from a terminal end thereof. The fixture is also provided with a generally planar silicon base. The base includes an attachment element receiving portion having the attachment element of the support member fixedly secured therein.